In subscription television, or pay television, programming signals of premium channels are typically transmitted in a scrambled form so that unauthorized viewers, that is, viewers who do not pay to receive the specific programming, are not able to view the transmitted signal on their television receivers. Authorized viewers are provided with means to descramble or decode the scrambled television signal and usually such descrambling means are capable of being selectively enabled to descramble specific channels. This enablement can be done remotely by the service provider or the CATV operator, by selective addressing of individual descrambling means associated with subscribers who paid to receive the scrambled programming.
A technique commonly used for television scrambling is that of sync-suppression wherein the RF level of the horizontal and vertical synchronizing pulses is suppressed to a level below that of the video so that the standard television receiver is unable to establish regular synchrony and instead erratically locks on RF peaks in the active video thereby creating unviewable picture on the television set. In addition, the ability of the television receiver to use the color reference burst associated with the horizontal synchronizing pulse is severely degraded thereby causing distorted color reproduction or disabling the color circuitry of the television set. Exemplary prior art sync suppression systems are disclosed in U.S. Pat. No. 3,184,537 to Court et al; U.S. Pat. No. 3,478,166 to Reiter et al; U.S. Pat. No. 3,530,232 to Reiter et al and U.S. Pat. No. 4,222,068 to Thompson. In these systems, the horizontal synchronizing levels are suppressed or reduced to gray level and an additional keying control signal is normally transmitted together with the scrambled television signal for controlling reconstruction or regeneration of the proper sync levels at the authorized descrambler, thereby providing unscrambled normal television signal at the receiver. For example, in U.S. Pat. No. 3,184,537 the audio subcarrier is amplitude modulated with a suitable sync insertion control signal. In U.S. Pat. No. 4,222,068 horizontal sync reconstruction at the authorized descrambler is effected by transmitting normal sync signals during the Vertical Blanking Interval ("VBI") of the television signal for enabling a timing circuit at the receiver to lock on the horizontal components thereof. The timing circuit is then used to gate the Horizontal Blanking Interval ("HBI") thereby facilitating the restoration of the horizontal synchronizing pulses in the composite baseband video signal.
Sync suppression systems normally operate in one of two ways: The first method, as in U.S. Pat. No. 3,184,537, is based on attenuating the modulated RF television signal during the HBI (and in some cases during the VBI as well) by a known fixed amount such as 6 Db. At the authorized descrambler, the signal is descrambled by attenuating the RF received signal during the active video portion by the same known amount. This method is known as "RF sync suppression" since the scrambling and descrambling processes occur at RF stages after the modulation process at the transmitter and before the demodulation process at the receiver respectively. The second sync suppression method is known as "Baseband Sync Suppression" wherein a known gated offset level is added to the baseband video signal during the HBI, thereby suppressing the horizontal synchronizing signal. At the authorized descrambler, an equal offset level is subtracted from the demodulated video signal during the HBI, thereby restoring the horizontal synchronizing signal levels to their normal values. An example of Baseband sync suppression system is disclosed in U.S. Pat. No. 4,222,068.
Both prior art RF and baseband sync suppression descrambling systems described above are typically accomplished by incorporating the timing and descrambling circuitry with a television tuner and demodulator, often termed a converter, that can cover the entire CATV band or over-the-air channels used by the subscription television service provider. In RF sync suppression systems, descrambling is effected by gated attenuation at the IF stage of the converter or at its fixed RF output channel frequency such as TV channel 3 or 4. In baseband sync suppression systems, descrambling is effected at the demodulated video stage of the converter and the resultant normal video signal is remodulated by means of a fixed frequency television modulator operating on channel 3 or 4. Thus, in both examples, the subscriber is equipped with a converter-descrambler device capable of tuning and descrambling a single channel at a time and converting it to channel 3 or 4 (output channel), whichever is unused by a local broadcast station. The subscriber's television set may permanently remain tuned to the converter-descrambler output channel while channel selection is done by means of the converter-descrambler tuner.
In a subscription television service, a plurality of scrambled channels may be transmitted simultaneously on several channels. However, as explained above, prior art converter-descramblers can only process one channel at a time. This means that subscriber households who are authorized to receive several premium scrambled channels, cannot receive these services descrambled simultaneously. So for example, if dad wishes to watch a "pay-per-view" event such as a championship fight, while the kids watch the Disney Channel on the second television set, the household would require two converter-descramblers. Furthermore, if mom wanted to record a movie on HBO at the same time, a third converter-descrambler must be employed in conjunction with the Video Cassette Recorder ("VCR"). This is clearly one of the disadvantages of prior art single channel descrambling techniques.
Most television households in America are equipped with "Cable ready" television sets or VCR's. The term "Cable ready equipment" should be understood as consumer television equipment with expanded frequency tuning capability that allows a subscriber to tune not only the off-air broadcast channel frequencies but also all CATV channels. A significant portion of the cable ready equipment sold in America contains additional features such as Multichannel Television Sound ("MTS") allowing reception of stereo TV programming or second audio programming ("SAP"), remote control to control sound level or sound muting as well as channel tuning. Still another feature of some cable ready television sets is the Picture-In-Picture ("PIP") display capability allowing the subscriber to simultaneously watch one program while observing additional video program(s) in a picture inset displayed within the main picture. When this cable ready equipment is connected directly to the cable drop entering the household, the subscriber is usually able to receive and record all non-scrambled channels without the aid of a converter and thus use his equipment's special features described above to receive such non-scrambled signals. The problem occurs when the CATV system employs signal scrambling as means of controlling access to the premium channels.
Normally in these circumstances, being fed by the converter-descrambler's output channel, the television set is permanently tuned to channel 3 (or 4) while the converter-descrambler is tuned to the desired channel. Thus, the remote control purchased with the television set is unused, and an additional remote control unit must be provided for the converter-descrambler. Moreover, many prior art baseband converter-descramblers cannot pass the composite MTS audio program material and thus the stereo signal and the SAP are lost despite the fact the television set may be capable of receiving MTS. Recent attempts to address this problem typically involve a stereo bypass operating mode as disclosed in U.S. Pat. No. 4,630,113 to Long, which unfortunately disables the remote volume control of the converter, or alternatively by dematrixing and reprocessing the stereo signal at the converter in order to effect volume control with resultant degradation of stereo separation and compander performance. Even prior art RF sync suppression descramblers tend to introduce MTS audio performance degradations since they pass the audio subcarrier with additional amplitude modulation at the horizontal line rate thereby causing AM to FM effects at the television receiver. These audio degrading effects are discussed in an article by J. O. Farmer entitled "Operational Characteristics of Modern Set-Top Terminals" published in the Technical Papers of the 33rd Annual Convention of the NCTA in Las Vegas, Jun. 3-6, 1984 and in the IEEE Transactions on Consumer Electronics, Vol. CE-30, pages 489-502, August 1984. Finally, the PIP feature of new television sets requires that the TV set be able to receive multiple channels simultaneously, a requirement that obviously cannot be met by current prior art converter-descramblers. Clearly, all these problems constitute yet another set of significant disadvantages of the prior art single channel descrambling techniques.
Collectively, the foregoing deficiencies discussed above are known in the industry as the "CATV consumer interface" problem. There is a growing body of evidence that these consumer interface problems present severe hardships on consumers and CATV operators alike. Specific evidence and indication of the public interest in this regard is expressed by legislation cited as the "Cable Ready Equipment Act of 1991" recently proposed by Senator Patrick Leahy in U.S. Senate Bill S. 2063, published in the Congressional Record--Senate, pages S18377-S18380, Nov. 26, 1991. Leahy's bill seeks to encourage solutions to the CATV consumer interface problems described above.
It may be easily understood that the most desirable solution to the problems discussed above is to provide the subscriber with a cable drop which carries all the channels to which he/she is entitled in unscrambled form ("in the clear"). The subscriber can then connect by proper broadband RF signal splitting means multiple cable ready television sets and VCR's and freely enjoy all of the consumer features that were purchased with that equipment. Since the CATV system may be carrying other channels not purchased by the subscriber, an effective means is required to limit a subscriber's access only to those channels and special events for which subscription fees have been paid.
If one attempts to employ prior art single channel descrambling techniques in order to provide each subscriber with simultaneous multichannel descrambling, one would have to provide each subscriber with a multiplicity of prior art descrambling means wherein each descrambler is tuned and dedicated to one scrambled channel. The outputs of such descramblers must then be assigned distinct channel frequencies and must be combined by means of a diplexer with all non-scrambled channels to form a broadband multichannel clear signal service entering the subscriber home. Clearly, the cost per subscriber for such an arrangement could reach the prohibitive value of the cost of one descrambler times the number of processed channels.
There thus exists the need for an effective low cost system for simultaneously descrambling an arbitrary subset of a plurality of scrambled channels, said arbitrary channel subset being the channels to which the subscriber has subscribed, and supply them together with the basic non-scrambled channels into the home in such a manner that all authorized channels are supplied "in the clear" and all scrambled channels not purchased by the subscriber are passed through into the home either in their original scrambled form, or preferably with additional security added to further deny any unauthorized access by "piratical" devices.